I have also a problem with nettles, it may be connected to this or be unrelated.
Most times I only collect mature plants to thresh before retting, some days there are only a few nettles sometimes dozens. After retting I sometimes end up with retted plants that will not stack. I ended up with 7 stacks of 5 to 12 nettles that were retted and dried but could not extract fibre as each stack was to small, other dried retted plants will stack and I think some nettles did.
The problem may have been caused by me, I had been experiments with a mod to use straw from other plants to make straw baskets and maybe this interfered with the nettle code.